1. Field of the Invention
The invention relates to data handling systems in particular to apparatus, systems and methods for interrogating, transmitting and receiving data and control signals between a central processor and a number of remotely located transducers and actuating devices.
2. DESCRIPTION OF THE PRIOR ART
Electronic data interface systems are employed to communicate information between a central processing unit and a plurality of remotely situated units. These sensors are largely analog due in part to the analog nature of physical phenomena. Digital central processing units comprising such devices as microprocessors can be used with the necessary interface circuitry to communicate with analog devices. Analog devices exhibit a greater sensitivity to noise pick-up than do digital devices making it more difficult to maintain measurement fidelity in noisy environments. Those sensors that are considered digital include devices having only two states or devices incorporating electronic conversion to a binary format. Digital devices while having a greater noise immunity than its analog counterpart often require more electrical lines for data transfer.
Prior art intercommunications between a plurality of analog sensors and a digital processor is disclosed in U.S. Pat. No. 3,059,228 to Beck et al. issued Oct. 16, 1962. Beck teaches a multi-wire system which simultaneously samples and holds a plurality of voltages. These voltages are later combined into a composite signal forming a series of analog voltages on a common lead. This composite signal is later successively digitized. The use of a single cable transmission system is taught by Penter in U.S. Pat. No. 3,132,329 issued May 5, 1964. Penter, as in Beck, requires that the remote device have its own power supply. Fergus in U.S. Pat. No. 3,678,512 issued on July 18, 1972 employs a single bi-direction transmission line using an analog-to-pulsewidth conversion encoding technique employing a trilevel interrogation voltage signal. In Fergus the length of time the transmission line is clamped to ground is indicative of the value of the sensed variable.
Demand responsive telemetry systems capable of selectively polling remote devices is shown in Higginbotham in U.S. Pat. No. 3,538,504 issued Jan. 15, 1968 using programmable remote processing units controlled by a central digital computer. In Rawson et al U.S. Pat. No. 3,566,365 issued Feb. 23, 1971 a central computer is again used to selectively poll outlying stations, each having a transducer, at a predetermined time and at a predetermined rate. Data is transmitted via a multi-wire cable using a 12-bit digital word. Both Higginbotham and Rawson while incorporating a sophisticated computer for control purposes still require separate external power sources at each test site.
Hadden in U.S. Pat. No. 3,717,858 issued Feb. 20, 1973 teaches a telemetering system having a single sensor located remotely from a base station where communication and power linkages are through a single two-way cable plus ground link. Analog data is encoded so that the repetition rate of a train of pulses is indicative of the quantity being sensed. The polarity of each pulse is of opposite voltage to the power signal supplied by the base station. Pulse width discrimination is accomplished by Hadden in U.S. Pat. No. 3,742,473 issued June 26, 1973 wherein a pulse generator, located at the base station charges a capacitor through a switching transistor such that the charging time is equal to the minimum pulse width to be discriminated. In Whiteside U.S. Pat. No. 4,103,337 issued on July 25, 1978 held by the assignee of present invention, teaches an analog to pulse width to digital data transmission system that can selectively or sequentially communicate with a plurality of remote sensors. Power, data, sensor activation and sensor reply signals are all sent via a single bi-directional line. Sensor interface circuitry is selectively activated upon decoding of an interrogation signal that is simultaneously received by all interfaces. The interrogation signal is comprised of a predetermined number of pulses. These pulses are sensed by a plurality of decoding units having shift registers. If the number of pulses corresponds to the number of storage locations minus one the appropriate sensor is then activated. The interface generates a low impedence to ground reply signal, the duration of which is indicative of the analog sensed value. The processor terminates the transmission of the data signal when it detects the low impedance to ground reply signal. In addition, the system utilizes power that is available from the interrogation pulse, thereby implicitly requiring that sufficient power be transferred to enable interface circuitry to encode the sensor information in the time between pulses.
As it can be seen most prior art tele-communications systems involve communications with remote sensors which transmit only data. Venema in U.S. Pat. No. 3,651,454 issued Mar. 21, 1972 discloses a multiplexing system for automotive application having both sensors; i.e. a data transmitter and data receiving devices. Sequential data transfer is accomplished utilizing a delay one-shot multivibrator responsive to a periodic clock pulse. Fergus, previously discussed, additionally teaches transmitting a coded series of pulses that is used to activate independently powered remotely located control receivers. The disclosed system is an improvement over the prior art discussed above.
It is an object of the invention to provide a multiplexing system capable of selectively communicating with a plurality of sensors and actuators via a single 2 wire communication link. A further object is to simplify the method of transfer of electrical power from a transmitting location to sensors and actuators and their respective interface networks. A still further object of this invention is the command capability of independent control of a number of actuators. Another object of the present invention is to provide for a refigurable and flexible multiplexing system having the capability of operating with the addition or removal of sensor and actuator units.